MIMO (Multiple Input Multiple Output) technologies have been widely used to improve data transmission rate, cell coverage and average cell throughput. MIMO uses a precoding technique, which adaptively controls a precoding vector based on feedback information received from user terminals (e.g. PTL 1).
Recently, LTE (Long Term Evolution)-Advanced standard is being developed for 4th generation system (4G), where the fairly aggressive target in system performance requirements have been defined, particularly in terms of spectrum efficiency for both downlink (DL) and uplink (UL) as indicated in the Sect. 8 of NPL 1. Considering the target of the cell-edge user throughput and the average cell throughput, which is set to be roughly much higher than that of LTE Release 8 (or Rel8), it seems that the coordinated transmission is necessarily included as a major candidate in LTE-Advanced techniques.
Coordinated multi-point transmission/reception is considered for LTE-Advanced as a tool to improve the coverage of high data rates, the cell-edge throughput and/or to increase system throughput in both high load and low load scenarios as described in the Sect. 8 of NPL 2. The point for coordinated multi-point transmission/reception can be used as a cell, base station, Node-B, eNB, remote radio equipment (RRE), distributed antenna, etc.
According to 3GPP RAN1 study item discussion, the additional feedback scheme for supporting coordinated multi-point transmission (CoMP) is needed and it was agreed that the feedback designed for CoMP should have good backward-compatibility to the feedback for LTE Release 8/9. In addition, the feedback design for CoMP should be consistent with the enhanced feedback framework for supporting single-cell dynamic switch between SU (single user)-MIMO and MU (Multi user)-MIMO transmission in LTE-Advanced as described in Sect. 7.3 of NPL 2.
In LTE Release 8/9, user equipment (UE) measures and feeds back the channel state information (CSI) to its serving cell. The serving cell is defined as a cell to transmit physical downlink control channel (PDCCH) to the UE as defined in Sect. 8.1.1 of NPL 2. The feedback CSI includes the UE recommended precoding matrix index (PMI) of Release-8 codebook, channel quality index (CQI) in terms of signal-to-interference plus noise ratio (SINR), and rank indicator (RI) for optimizing a single-cell SU-MIMO transmission (e.g., throughput maximization). The eNB directly use the feedback PMI/RI to carry out the precoding transmission. The feedback CQI is used for channel-dependent scheduling.
For LTE-Advanced, it is required that the single-cell dynamic SU/MU-MIMO transmission is supported, by which SU-MIMO and MU-MIMO transmission can be dynamically switched e.g., TTI-by-TTI or frame-by-frame, which is hereafter referred to as single-cell SU/MU-MIMO transmission. An enhanced feedback scheme for single-cell dynamic SU/MU-MIMO has been proposed in NPL 3 and NPL 4, where the long-term auto-correlation (or self-correlation) channel matrix of the serving cell is fed back in addition to the short-term CQI/PMI/RI. The long-term (e.g., 50 ms, 100 ms or 1 s) and/or wideband (e.g., full system bandwidth) auto-correlation channel matrix is to capture the semi-static and/or frequency-non-selective correlation characteristics of channel; the short-term (e.g., 1 ms, 5 ms or 10 ms) and/or subband CQI/PMI/RI is used to represent instantaneous and/or frequency-selective channel. Based on the enhanced feedback, the eNB can decide the precoding vector for SU-MIMO and MU-MIMO, respectively. The SU-MIMO and MU-MIMO transmission can be dynamically switched (e.g., TTI-by-TTI) by choosing the higher achievable data rate based on the long-term and short-term feedback.
For LTE-Advanced DL CoMP, NPL 5 discloses a scheme to feed back per-cell long-term and short-term PMI/RI/CQI. It is a simple extension of enhanced feedback for single-cell SU/MU-MIMO transmission. The eNB uses the per-cell feedback CSI to select the precoding vector for each cell. The per-cell long-term feedback includes the respective auto-correlation channel matrixes of the serving cell and neighbor cell (regarded as a cooperating cell for CoMP). The per-cell short-term PMI/RI/CQI feedback is selected in the same way as single-cell transmission at the serving cell and neighbor cell. However, such a precoding vector selection for individual cell is separate precoding and the inter-cell interference is not considered. Therefore, the scheme for DL CoMP feedback as described in NPL 5 can only support separate precoding with SU/MU-MIMO transmission. Similarly, PTL 2 discloses a multi-cell coordinated transmission method which employs the feedback scheme supporting the separate precoding, which is similar to the scheme for DL CoMP feedback of NPL 5 (see PTL 2, paragraph 0106, col. 15).    {NPL 1} 3GPP TR 36.913 v9.0.0, Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE-Advanced), December 2009. http://www.3gpp.org/ftp/Specs/archive/36_series/36.913/.    {NPL 2} 3GPP TR 36.814 (V2.0.1), “Further Advancements for E-UTRA Physical Layer Aspects,” http://www.3gpp.org/ftp/Specs/archive/36_series/36.814/.    {NPL 3} R1-101062, Huawei    {NPL 4} R1-1000852, Ericsson    {NPL 5} R1-094243, NTT DOCOMO    {PTL 1} JP2009-10752A    {PTL 2} JP2010-45783A